Open-ended coaxial sensors are routinely used in measuring the dielectric permittivity of materials using the reflection of high frequency radiation from the interface between the open end and the material under investigation. This probe measures the scattering parameter (S11) and uses it to derive the reflection coefficient from the interface. The reflection coefficient is related to impedance difference between the two media in the interface and is given by: γ=(Z1−Z2)/(Z1+Z2), where Z1 and Z2 refer to the impedances of the medium under investigation and the coaxial probe. Knowing the impedance of the open ended coax and the reflection coefficient allows calculating the impedance and thus the dielectric permittivity of the medium under investigation.
A problem with using a conventional open-ended coaxial probe is its shallow depth of investigation. As a result the sensor is quite good and easy to use for measuring the dielectric properties of liquids where there is a good contact between the end of the probe and the liquid. Using the probe to measure the dielectric properties of solids is more challenging since a small gap between the probe and the solid can lead to large errors. Currently there exists a commercial open-ended coaxial sensor for dielectric measurement of liquids. The application of any open ended coaxial probe to measure solid samples is limited to flat surfaces at best.